This invention relates to an improvement in a polyether imide or polyimide liquid crystal-aligning coating capable of controlling the alignment and pretilt angle of liquid crystals.
Further, the present invention relates to a novel diamino compound useful as a raw material of a polyimide for the aligning coating for liquid crystal display elements affording a large pretilt angle, and a dinitro compound and diol compound each as an intermediate for the diamino compound.
2. Description of the Related Art
As to conventional liquid crystal display elements used for watches, electronic calculator, etc., twisted nematic (hereinafter abbreviated to TN) mode having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90.degree. on the surfaces of a pair of two upper and lower electrode substrates has been mainly employed. However, as to such a display mode, at the time of a high duty drive, the contrast is low and the viewing angle is narrow due to cross-talk; hence the mode has been insufficient for improving the display quality and display area. In recent years, a liquid crystal display device making use of a super twisted birefringence effect has been announced (T. J. Scheffer and J. Nehring, Appl. Phys. Lett , 45 (10), 1021 (1984)). Since then, a liquid crystal display element making use of a super twisted nematic (hereinafter abbreviated to STN) mode wherein the aligning direction of nematic liquid crystal molecules is twisted by 180.degree. to 300.degree. between the upper and lower electrode substrates has been developed, and a liquid crystal display device having a satisfactory display quality even in the case of a liquid crystal display of a large picture surface has been under development. In the case of an aligning membrane used for such a device, in order to not only merely align the liquid crystals but also to improve the response properties of the device and ensure the bistability thereof, a definite angle (hereinafter abbreviated to "pretilt angle") should be provided between the substrate plane and the liquid crystal molecules. Further, as the twist angle is increased, it is preferred to increase the pretilt angle. As to the interface processing on the electrode substrates, in the case of a relatively small twist angle (180.degree. to 200.degree. twist) among the above twist angles, a currently generally used cell provided with an aligning membrane having an interface having a pretilt angle (abbreviated to .theta.) of 5.degree. or less may be sufficient. However, in the case of a mode having a twist angle of 210.degree. to 300.degree. at which a better display quality is afforded, it is necessary to use an interface having a large pretilt angle (5.degree.&lt;.theta..ltoreq.30.degree.); hence a liquid crystal display cell provided with an aligning coating satisfying these pretilt angles is required.
In the case of currently used polyimide aligning coatings for TN mode, the pretilt angle of display cells prepared on a commercial scale has a limitation of 5.degree..
Further, polyimide aligning coatings for STN mode having a larger pretilt angle are present, but the coatings have a problem in the aspect of the stability and reproducibility of the tilt angle over the total region of the cell substrate having a broad display area. In order to certainly obtain a large pretilt angle, formation of a thin coating by means of oblique vacuum deposition such as SiO is the best process currently carried out.
However, as to the formation of a thin coating by means of vacuum deposition, when the coating is commercially mass-produced, it is disadvantageous in the cost in the aspect of production device. Thus, it has been earnestly desired to obtain alignment and a large pretilt angle by subjecting an organic thin coating to surface treatment by rubbing in the same manner as that employed in the case of conventional TN mode and realize the stability and reproducibility of the tilt angle.
On the other hand, Japanese patent application laid-open No. Sho 61-240223/1986 discloses a liquid crystal display element provided with a liquid crystal-aligning coating using a polyimide resin having repetition units expressed by the formula ##STR2## Further, the above laid-open application discloses a working example using as a raw material for the above polyimide, a diamine expressed by the formula ##STR3##
However, the polyimide aligning coating obtained by using the diamine compound has a drawback that a large pretilt angle cannot be obtained, as described in Comparative example mentioned later. Thus a novel diamino compound used as the raw material for the organic aligning coating has been desired.